Donald Trump could draw the United States into nuclear war, Hillary Clinton warned. Mrs Clinton would plunge the country into a constitutional crisis, he said.

As the caustic presidential race lurches toward the finish line, each candidate is aggressively casting the other as a catastrophic choice for the White House. Making an affirmative case about their own qualifications and vision has become a secondary priority.

It is an ugly conclusion to a contest featuring two of the most unpopular presidential candidates in modern American politics.

The sexual assault accusations that have trailed Mr Trump in the race's closing weeks and a new FBI review into Mrs Clinton's email habits seem likely to only reinforce the public's negative perceptions, leaving the candidates to essentially argue to voters that they're the best of two unappealing options.

"I would rather be here talking about nearly anything else," Mrs Clinton said on Tuesday during a rally in Florida where she levelled a series of attacks on Mr Trump's character and preparedness for the White House.

"But I can't just talk about all of the good things we want to do."

Indeed, Mrs Clinton's speeches in this final full week of campaigning have overwhelmingly focused on Mr Trump. On Monday, she warned against giving Mr Trump the authority to order a nuclear attack, bringing along a former nuclear launch officer to bolster her point.

"Imagine his advisers afraid to tell him what he doesn't want to hear, racing against his legendarily short attention span to lay out life-and-death choices too complex to be reduced to a single tweet," Mrs Clinton said Monday in Ohio. "Then imagine him plunging us into a war because somebody got under his very thin skin."

After spending much of the summer and fall tearing Mr Trump down, Mrs Clinton had planned to close the campaign on a more positive note. She talked about giving Americans something to vote for, not just against. And with public opinion polls showing her with solid leads in most battleground states, she started talking about healing divisions and unifying the country after the election.

But her advisers say they saw polls tighten even before the FBI launched its new email review. The campaign now believes she needs to make a last push to define Mr Trump as an unacceptable choice in order to seal the deal with persuadable voters.

On Tuesday, Mrs Clinton focused on Mr Trump's demeaning and predatory comments about women, calling him a "bully". This time she brought with her former Miss Universe Alicia Machado. Mr Trump criticised Ms Machado for gaining weight after winning the 1996 contest.

Mr Trump's campaign rhetoric has always been dark, full of searing depictions of a crumbling nation, and he has not been shy about going negative on Mrs Clinton. He routinely calls her "Crooked Hillary" and "the most corrupt person ever to run for the White House".

But Mr Trump, too, has stepped up his broadsides after the last weeks of October handed him a pair of potentially potent political gifts: the projected Obamacare premium rate hike and FBI director James Comey's letter revealing that agents are reviewing emails that may be connected to Mrs Clinton's private server.

His campaign sees the latter in particular as an opportunity to reinforce questions about Mrs Clinton's trustworthiness and remind voters that sending Mrs Clinton to the White House could lead to the return of the scandals that trailed Bill Clinton's presidency in the 1990s.

"She would be under protracted criminal investigation and probably a criminal trial, I would say," Mr Trump said during a rally in Michigan on Monday. "So we'd have a criminal trial of a sitting president."

Campaigning Tuesday in Pennsylvania, a state in which Mr Trump has directed an abundance of time and resources, he and his running mate Mike Pence delivered their most full-throated criticism yet of President Barack Obama's health care law.

Her campaign has several commercials out that directly question whether Mr Trump would launch a nuclear attack. The ads feature clips of him saying he likes to be unpredictable and would "bomb the (expletive) out of them".

She also doubled down on her argument that Mr Trump's offensive comments about women, as well as his boasts about touching women without their permission, disqualify him from the White House.

A 60-second ad that features Mr Trump in his own words over the years concludes: "Anyone who believes, anyone who says, anyone who does what he does, is unfit to be president."

Meanwhile, Mr Trump's ads reinforce his message that the country risks doom if it doesn't change directions by electing him. "Hillary Clinton will keep us on the road to stagnation," a narrator says in one of his latest ads.